Thermoplastics are widely used for extruded and molded products because of the high speed at which they can be extruded or injected into molds. They may be heated to high temperatures where their viscosity is lower, so they fill molds more quickly and completely. The polymers are also generally less expensive because they do not contain cross-linking agents which are themselves costly and frequently require an extra process to incorporate into the polymer. Overall cycle times and extrusion speeds are higher because no secondary cross-linking step is necessary.
Thermoplastic polymers, however will re melt when exposed to heat, experience creep, and will not return to their original shape when stretched or stressed and may have lower solvent resistance.
Thermosetting polymers have desirable properties such as heat resistance, abrasion resistance, low creep and higher modulus, elastic recovery and chemical resistance because of the ties or cross links between the polymer chains but typically must be processed at lower speeds and temperatures to prevent premature cross linking and then undergo a separate cross linking step. It should be noted that high modulus can be equated to stiffness in that it takes more energy or force to bend a profile or article but is not the same as rigid which means very high modulus. Rigid articles can be produced by the methods of the invention but are typically made with less expensive not cross-linked stiffer plastic such as ABS and PVC. In certain instances, for example chemical resistance, cross-linked plastics may be desired, however.
It should be understood that in the context of this invention, un cross-linked polymers are also called thermoplastic and cross-linked polymers are also called thermosetting. Some polymers may be processed as thermoplastic and also can be made to be thermosetting by the addition of a cross linking agent or by undergoing special processing steps while other polymers are difficult or impossible to cross link and are always thermoplastic. In the context of this application thermosetting polymers are defined as those that have cross linking agents and thermoplastic polymers are defined to be polymers that cannot be crosslinked or that do not have cross linking agents added.
The conventionally accepted definition for thermoplastics has been that a thermoplastic polymer can always be heated to soften the thermoplastic polymer. These polymers may be linear and in some cases have some branching of the polymers.
The conventionally accepted definition for thermosetting plastics or polymers is that a thermosetting polymer, once cured, can not be further shaped by heat because such polymers are space network polymers and are highly cross-linked to form rigid three-dimensional molecular structures, such as in urea-formaldehyde or phenol-formaldehyde resins.
The cross-linking or curing of the curable polymers of the invention causes them to have properties of conventional thermosetting resins, and accordingly, the curable polymers of the invention are called thermosetting in the context of this invention.
Co-extruded and co-molded products and the like are well known in the art. Films, electrical cables, and articles where different colors are desired on different parts of the article are often co-extruded. Most heat sealable food packaging is made in the extruded or blown film process with several layers for sealing, oxygen and moisture barrier, strength and outside layers suitable for printing being processed in one step. Many articles are manufactured by co-injection or over molding. Other articles are molded with stiff thermoplastic components in their structure and then flexible thermoplastics areas for hinges, snap fit attachments and the like.
EP 1 072 380 A2 discloses a co-extruded profile that is combined with molded profiles to obtain an article with a soft portion and a rigid portion, both uncured. PCT WO 96/01171, WO 97/08005, EP 0 792 217 B1 and U.S. Pat. No. 6,169,276 disclose examples of co-extruded profiles.
Co-extruded or blown films must typically include “tie layers” or adhesives to adhere the different polymers together. Due to the high temperatures necessary to melt some of the polymer layers and provide high throughputs, cross-linkable polymers are not used. Tie layers add expense while providing little or no added benefit to the construction.
Thermosetting polymers and thermoplastics may be co molded successfully. A commercial pan handle is produced by injecting a thermosetting component onto the walls of the mold and then injecting a thermoplastic core. If carefully controlled the hotter thermoplastic part may speed the cure of the thermosetting portion. The article must still be heated in the mold to complete the cross linking reaction and then cooled, before being ejected from the mold, causing long cycle times.
Moisture-curable polymers are available and have been used. Such polymers, based on polyethylene, are commercially available as Aquathene® from Equistar Chemicals, Aqualink® from AT plastics and Silink® from Union Carbide. Many olefins or other polymers that can be crosslinked with organic peroxides can be crosslinked with moisture. Polymers that can be crosslinked with peroxide can be made to be moisture-curable by grafting 0.1 to about 5% vinyl trimethoxysilane, for example, on their backbone with organic peroxide in an extruder of suitable length at a temperature over the decomposition temperature of the peroxide and with the addition of 0.1 to about 1% peroxide, 0.1 to about 3% dibutyl tin dilaurate, carboxylate of zinc, lead, iron or colbalt or certain base or acid catalysts. Such mixtures of moisture cure systems are commercially available from OSI Inc. as Silcat R®. Other combinations of silanes, peroxides and catalysts are available or can be custom blended for specific applications. The polymer may be grafted in the extruder and then injected directly in the production of the article of manufacture or can be pelletized for later use. Other moisture cure systems utilizing various chemistries are also available. Moisture curing takes place when the hydrolyzable silane groups are hydrolysed under the influence of water, resulting in the splitting-off of alcohol and the formation of silanol groups. In a second step the silanol groups are cross-linked by a condensation reaction. In both steps a silanol condensation catalyst is used as a catalyst. Details may be found in British Polymer Journal 21 (1989), pp. 117 to 123.
In medium voltage power cables up to three layers, the inner and outer containing carbon black to provide electrical conductivity, are co-extruded and all cured with moisture or peroxide. PCT WO 00/68957, WO 00/36612, U.S. Pat. Nos. 6,080,810 and 5,716,574 are examples and incorporated herein by reference. EP 0 926 944 A1 discloses a moisture cable and teaches that moisture cure can not by used with peroxide cure.
Microwave, dielectric or radio frequency heating and curing of polymers is known in the art. U.S. Pat. No. 5,721,286 discloses a method for curing polymers with microwave energy in a continuous fashion. U.S. Pat. No. 5,166,484 discloses an improved method of continuously curing rubber parts and the like with more uniform heating. U.S. Pat. No. 6,075,063 discloses a method for forming a component of high rigidity by foaming inside hollow cavities of the rigid article using microwave energy. PCT international publication WO 97/36965 discloses forming a composite by incorporating a electrically conductive pattern between the layers that generate heat that melts adhesive layers when exposed to microwave, dielectric or radio frequency energy. While microwave curing has many advantages it is rarely used because of the higher initial cost, the need to tailor the system to the article being cured and the fact that current manufacturing equipment in use has a long life and can be repaired and rebuilt easily. The advantages gained by utilizing the methods in the invention, and the likelihood that new production lines would be needed for the improved products, far outweigh the disadvantages disclosed, however.
None of the prior art discloses a moisture-curable or microwave, dielectric or radio frequency curable polymer composition that is co-extruded, calendered or co-molded with a thermoplastic polymer.